Abstract

Magnetic and structural properties of nanocrystalline low-doped cobaltites with particle size of 8, 13, 23, and 50 nm, prepared by the glycine-nitrate method, were investigated in temperature range 5–320 K, magnetic field up to 50 kOe and under hydrostatic pressure up to 10 kbar. With particle downsizing, a noticeable expansion of unit cell, with concomitant changes in the rhombohedral structure toward the cubic one was observed. It was found that the increased surface-disorder effect strongly suppresses the ferromagnetic state in nanoparticles leading to a decrease, by factor of about 2, both in spontaneous magnetization, , and Curie temperature, , when particle’s size decreases from 23 to 8 nm. The effective magnetic moment was found also to decrease distinctly due to the strong interdependence between Co–O–Co interactions and Co spin state. The size-induced magnetic disorder drives the nanoparticles to a dominant glassy behavior for 8 nm particles. This is evidenced by the fact that the freezing temperature varies with magnetic field in a strict conformity with the de Almeida–Thouless law for spin glasses and also by the observation of characteristic slowing down in the spin dynamics. The applied pressure suppresses , , and coercive field, like it is observed for bulk . Nevertheless, in nanoparticles the pressure effect on is noticeably stronger, while diminishes with pressure much slower then in bulk material.

Received 28 January 2010Accepted 01 July 2010Published online 17 September 2010

Acknowledgments:

This work was supported in part by the Polish Ministry of Science and Higher Education under a research project No. N N202 1037 36 and co-financed from EU resources under Innovative Economy Programme project UDA-POIG.01.03.01-00-058/08-00.